Over the last decade or so, the utility of inorganic nanoparticles as additives to enhance polymer performance has been well established. Ever since the seminal work conducted at Toyota Central Research Laboratories, polymer-layered material nanocomposites have generated interest across various industries. The unique physical properties of these nanocomposites have been explored by such varied industrial sectors as the automotive industry, the packaging industry, and plastics manufacturers. These properties include improved mechanical properties, such as elastic modulus and tensile strength, thermal properties such as coefficient of linear thermal expansion and heat distortion temperature, barrier properties, such as oxygen and water vapor transmission rate, flammability resistance, ablation performance, and solvent uptake. Some of the related prior art is illustrated in U.S. Pat. Nos. 4,739,007, 4,810,734, 4,894,411, 5,102,948, 5,164,440, 5,164,460, 5,248,720, 5,854,326, and 6,034,163.
Nanocomposites can be formed by mixing polymeric materials with intercalated layered materials, which have one or more foreign molecules or parts of foreign molecules inserted between platelets of the layered material. Although any amount can be used, the physical property enhancements for these nanocomposites are typically achieved with less than 20 vol. % addition, and usually less than 10 vol. % addition of the inorganic phase, which is typically layered materials or organically modified layered materials. Although these enhancements appear to be a general phenomenon related to the nanoscale dispersion of the inorganic phase, the degree of property enhancement is not universal for all polymers. It has been postulated that the property enhancement is very much dependent on the morphology and degree of dispersion of the inorganic phase in the polymeric matrix.
The layered materials in the polymer-layered material nanocomposites are ideally thought to have three structures: (1) layered material tactoids wherein the layered material particles are in face-to-face aggregation with no organics inserted within the layered material lattice, (2) intercalated layered materials wherein the layered material lattice has been expanded to a thermodynamically defined equilibrium spacing due to the insertion of individual polymer chains, yet maintaining a long range order in the lattice, and (3) exfoliated layered materials wherein singular layered material platelets are randomly suspended in the polymer, resulting from extensive penetration of the polymer into the layered material lattice and its subsequent delamination. The greatest property enhancements of the polymer-layered material nanocomposites are expected with the latter two structures mentioned herein above.
There has been considerable effort towards developing materials and methods for intercalation and/or exfoliation of layered materials and other layered inorganic materials. In addition to intercalation and/or exfoliation, the layered material phase should also be rendered compatible with the polymer matrix in which they are distributed. The challenge in achieving these objectives arises from the fact that unmodified layered material surfaces are hydrophilic, whereas a vast number of thermoplastic polymers of technological importance are hydrophobic in nature. Although intercalation of layered materials with organic molecules can be obtained by various means, compatibilizing these splayed layered materials in a polymer matrix for uniform distribution still poses considerable difficulty. In the industry, the layered material suppliers normally provide just the intercalated layered materials and the end users are challenged to select materials and processes for compatibilizing these layered materials in the thermoplastics of their choice. This selection process involves trial and error at a considerable development cost to the end users. Since layered material intercalation and compatibilization in the matrix polymer usually involve at least two distinct materials, processes, and sites, the overall cost of the product comprising the polymer-layered material nanocomposite suffers.
A vast majority of splayed layered materials are produced by interacting anionic layered materials with cationic surfactants including onium species such as ammonium (primary, secondary, tertiary, and quaternary), phosphonium, or sulfonium derivatives of aliphatic, aromatic or arylaliphatic amines, phosphines and sulfides. These onium ions can cause intercalation in the layered materials through ion exchange with the metal cations present in the layered material lattice for charge balance. However, these surfactant molecules may degrade during subsequent melt processing, placing severe limitation on the processing temperature and the choice of the matrix polymer. If the final product is to be coated out of a solvent-borne phase, compatibility of the intercalant with the proper solvent may also impose restrictions on the choice of the intercalant. Additionally, these surfactants can act as lubricants and negatively impact the potential enhancement of properties such as modulus and strength of the nanocomposite.
Intercalation of layered materials with a polymer, as opposed to a low molecular weight surfactant, is also known in the art. There are two major intercalation approaches that are generally used—intercalation of a suitable monomer followed by polymerization (known as in-situ polymerization, see A. Okada et. Al., Polym Prep., Vol. 28, 447, 1987), or monomer/polymer intercalation from solution. Poly(vinyl alcohol) (PVA), polyvinyl pyrrolidone (PVP) and poly(ethylene oxide) (PEO) have been used to intercalate the layered material platelets with marginal success. As described by Levy et al., in “Interlayer adsorption of polyvinylpyrrolidone on montmorillonite”, Journal of Colloid and Interface Science, Vol 50 (3), 442, 1975, attempts were made to sorb PVP between the monoionic montmorillonite layered material platelets by successive washes with absolute ethanol, and then attempting to sorb the PVP by contacting it with 1% PVP/ethanol/water solutions, with varying amounts of water. Only the Na-montmorillonite expanded beyond 20 Å basal spacing, after contacting with PVP/ethanol/water solution. The work by Greenland, “Adsorption of poly(vinyl alcohol) by montmorrilonite”, Journal of Colloid Science, Vol. 18, 647-664 (1963) discloses that sorption of PVA on the montmorrilonite was dependent on the concentration of PVA in the solution. It was found that sorption was effective only at polymer concentrations of the order of 1% by weight of the polymer. No further effort was made towards commercialization since it would be limited by the drying of the dilute splayed layered materials. In a recent work by Richard Vaia et al., “New Polymer Electrolyte Nanocomposites: Melt intercalation of polyethyleneoxide in mica type silicates”, Adv. Materials, 7(2), 154-156, 1995, PEO was splayed into Na-montmorillonite and Li-montmorillonite by heating to 80° C. for 2-6 hours to achieve a d-spacing of 17.7° A. The extent of intercalation observed was identical to that obtained from solution (V. Mehrotra, E. P. Giannelis, Solid State Commun., 77, 155, 1991). Other work (U.S. Pat. No. 5,804,613) has dealt with sorption of monomeric organic compounds having at least one carbonyl functionality selected from a group consisting of carboxylic acids and salts thereof, polycarboxylic acids and salts thereof, aldehydes, ketones and mixtures thereof. Similarly, U.S. Pat. No. 5,880,197 discusses the use of an intercalating monomer that contains an amine or amide functionality or mixtures thereof. In both these patents, and other patents issued to the same group, the intercalation is performed at very dilute layered material concentrations in a medium such as water, leading to a necessary and costly drying step, prior to melt processing.
Recently use of block copolymers has been disclosed in U.S. Pat. Nos. 6,767,951 and 6,767,952 and U.S. Patent Appl. No. 20030100656 A1 for intercalation of smectite clays. In these disclosures intercalation has been reported to have been achieved during melt processing with or without any other intercalating agents. These patents teach of specific block copolymers, which can further compatibilize the clays in various matrix polymers.
However, a survey of the art makes it clear that there is in general a dearth of guidelines for the selection of the intercalating agents for a wide variety of matrix polymer and layered material combinations. In addition, it is desirable to have multifunctional addenda, so that their relative amount in the nanocomposite is small and therefore has minimum deleterious effect, if any, on the properties of the nanocomposite. There is a critical need in the art to identify such addenda. There is also a critical need in the art for a comprehensive strategy for the development of better materials and processes to overcome some of the aforementioned drawbacks.